Scuttling through deep sea caves far from the sun's light dwells a creature with a form so bizarre it feels alien. The Mesmerantula walks upon six crustacean-like legs and drags behind it a tail covered in sharp spines. Their head resembles a gelatinous orb with bright yellow eyes and a vertically split mouth. This unsightly appearance is thanks to their shapeshifting biology and diet. You see, the Mesmerantula takes “you are what you eat” to a whole new level. These shapeshifting bottom feeders' appearance is directly related to what they consume. In fact scientists will even use Mesmeantulas as bio-indicators, looking at their appearances for clues on how prevalent certain species are in an area. While most of these creatures act as bestial scavengers, others become blessed with higher levels of thought. This happens when a Mesmerantual consumes a humanoid. The diet of humanoid flesh not only affects their brain but like everything they eat it affects their physical forms. Which is why sentient Mesmerantulas orb-like heads extend into a humanoid torso and head that is even equipped with two arms ending in long gnarled claws. The type of humanoid they eat is another factor in how they look. Variations causing more exaggerated jellyfish qualities if Kandjii are consumed or more fur like forms of a Scruttle. The goals and temperament of sentient Mesmerantulas depend on the deceased humanoid. Some seek out friendship with humanoids while others may become maleficent monsters. Luring individuals into their caves with bioluminescent lights to consume their flesh and more memories. It’s because of this unknown element and their odd appearance that most humanoids avoid Mesmerantulas entirely, viewing spotting one near a town or village as a foul omen. 

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Credit to Leia, linnywines8014, and JJMonsterJ for some lore ideas.